Technical Abstract:
Nature of crops and management systems profoundly affect fungal communities in the rhizosphere of perennial crops. Fungal populations in soil are involved in organic matter transformation and nutrient cycling. A field experiment was established during 1999 at the Tropical Crop Institute, San Martin-Tarapoto Peru with cacao (Theobroma cacao L.). Banana (Musa spp.) and Guaba (Inga edulis) were also planted as shade crops along with cacao. Dynamics of fungal population associated with cacao rhizosphere was investigated. The field area was divided into three blocks and in each block; three sites were selected at random to obtain soil samples. At each site, soil samples were obtained at three depths (0-20 cm, 20-40 cm, 40-60 cm) and samples were mixed according to depth and one composite sample was selected for fungal determination. Soil samples were obtained during growing seasons of 1999, 2002, 2003 and 2004 for determination of fungal dynamics.Fungal detection was carried at ICT Phytopathology Laboratory. Serial dilution with inverted plate technique was used to differentiate fungus genus and fungal colonies were registered as colony forming units per gram of soil (cfu/g soil). The fungal identification was carried out at level of genera using the Barnett, Barron and Ellis key. The fungal population density in the cacao rhizosphere decreased from 1999 (58.2 x 103 cfu/g) until 2003 (10.2 x 103 cfu/g). However, fungal population increased significantly during 2004 (29.5 x 103 cfu/g). The population variations over time showed a quadratic tendency (r2= 0.902). The fungal density was highest in the first 20 cm of soil depth (15.1 x 103 cfu/g), and decreased significantly with increasing soil depth.Overall totally, 26 fungal genara were identified during the four years of evaluation: Phytophthora, Fusarium, Verticillium, Rhizoctonia and Cephalosporium were identified as potential pathogens of cocoa and banana and 17 genara were saprophytes. The predominant genera during four years of evaluation were: Penicillium, Aspergillus, Paecilomyces, Trichoderma, and Fusarium (17.5 x 103, 6.1 x 103, and 3.9 x 103, 1.5 x 103 and 1.3 x 103 cfu/g, respectively). The developments of fungal populations were favored by the soil conditions at 0 - 20 cm of depth. The soil under cacao was sandy loam in texture, with more than 60% of sand, low pH (4.40 a 4.58), and low content of organic matter (< 1.5%). The weather conditions, with high temperatures and heavy rain fall, also favored the development of fungus.